1. Field of the Invention
The present invention relates to a photothermographic material. More specifically, the invention relates to a photothermographic material which exhibits improved film surface strength.
2. Description of the Related Art
In recent years, in the field of films for medical imaging, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light-sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light-sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described in many documents. In particular, photothermographic materials generally have an image forming layer in which a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. Further, the Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system using photothermographic materials that has been made commercially available.
Thermal developing processing does not require the processing solutions used in wet developing processing, and has an advantage in that processing can be carried out easily and rapidly. However, on the other hand, there are particular problems to be solved because the photothermographic materials contain all components necessary for image formation in coated layers in advance and contain unreacted components or reaction products after image formation in the film. One is a problem concerning storage stability of the photothermographic material such as storage stability prior to use for image forming processing after production and storage stability of an image after image formation. Another is a problem concerning physical strength of the coated film such as being brittle or susceptible to defects.
As a means for improving image storage stability, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-352624 discloses the use of a non-photosensitive silver salt in a non-photosensitive layer on a side having an image forming layer. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein. However, there are problems concerning further deterioration of film physical properties caused by an increase in total layer thickness accompanying the increase in an amount of binder for including the non-photosensitive silver salt in the non-photosensitive layer or an additional layer.
As a means for improving film physical properties, for example, JP-A Nos. 9-146220, 11-228698, and 2003-262934 disclose the use of a non-bleaching dye technique instead of using a bleaching dye method as an antihalation dye technique. Various components such as base generating agents or radical generating agents used for the bleaching dye method are unnecessary and therefore result in decreasing the burden on the film, which is favorable for the film physical properties. However, there are problems such as occurrence of color unevenness due to color transfer of water-soluble dyes when they are contacted therewith.
Moreover, in order to provide resistance to defects on the film surface and prevent adhesion or color transfer during stacking, it is well known in the art to provide surface roughness by adding a matting agent in a surface layer of silver halide photographic materials.